Recently, the widespread use of the Internet makes content distribution more popular where content, including moving pictures and audios, is distributed and played back via an IP (Internet Protocol) network. In the distribution of this kind, in order to improve the transmission efficiency of for example moving pictures, highly efficient compression encoding scheme based on the inter-frame prediction is used in many cases. In those schemes, predictive parameters, acquired by predicting the encoded images from the temporally preceding and following frames, and predictive residual image data are encoded to reduce the amount information of temporally correlated moving picture data. In addition, the high compression and encoding of predictive residual image data through transform-coding and quantization enables content data to be transmitted using a small amount of transmission bandwidth.
Typical exemplary transmissions use compression and encoding schemes such as MPEG (Moving Picture Experts Group)-1, MPEG-2, and MPEG-4. In those compression encoding schemes, inter-frame prediction is performed for an input image frame for each fixed-size rectangular area, called a macro block, through motion compensation, and the obtained motion vectors and the signal data, generated by compressing the predictive residual image data through the two-dimensional discrete cosine transform and quantization, are subjected to variable-length coding.
Effective compression encoding schemes based on the inter-frame prediction, such as AAC (Advanced Audio Codec), are also available for audios to enable content distribution that uses the transmission bandwidth efficiently.
Encoded data encoded by an encoder is stored in a storage device in a file format described in Non-Patent Document 2 or Non-Patent Document 3 that will be described later. The encoded data is converted to packets by the distribution server using a method such as RTP (Real-Time Transport Protocol) described in Non-Patent Document 1, which will be described later, for distribution to a client via an IP network. The client obtains the encoded data from the received packets and decodes the data to reproduce videos and audios.
On an IP network, there is a possibility that distributed packets are lost. In particular, the possibility of packet loss is higher when the network is wireless.
And, when even a part of content data is lost, a client cannot completely decode videos and audios with the result that the videos or audios are disturbed by interferences or noises or sometimes cannot be reproduced at all. This effect is particularly obvious in encoded data based on the inter-frame prediction, and interferences or noises propagate in the time direction.
To restore lost packets, the following methods are used.                Retransmission        FEC (Forward Error Correction)        Multiple-packet transmission        
Retransmission is a method for requesting the distribution side to redistribute a packet, which is lost on the client side, to re-distribute the packet. Note that the retransmission cannot be used on a transmission line that has no uplink.
The FEC is a technique in which the distribution side transmits encoded data and the FEC data (error correction data) in advance and, when a packet is lost, the FEC data is calculated to restore the lost packet.
The multiple-packet transmission is a method in which a packet containing the same information is transmitted multiple times to reduce the probability of packet loss.
Another known configuration is that multiple encoders are used to generate the encoded data of audio data at different compression rates at the same time for transmission over a packet-based network such as an IP network (for example, Patent Document 1 that will be described later).    Non-Patent Document 1: IETF RFC 1889 “RTP: A Transport Protocol for Real-Time Application”    Non-Patent Document 2: ISO/IEC 14496-12:2003: “Information technology •Coding of audio-visual objects—Part 12: ISO base media file format”    Non-Patent Document 3: ISO/IEC 15444-12:2003 “Information technology—JPEG 2000 image coding system—Part 12: ISO base media file format”    Patent Document 1: Japanese Patent Publication Kokai JP-A 2003-318851 (FIG. 1)